Two-electron Oxygen Reduction Research Articles

Piezo-catalysis Mechanism Elucidation by Tracking Oxygen Reduction to Hydrogen Peroxide with In-situ EPR Spectroscopy.

For piezoelectric catalysis, the catalytic mechanism is a topic of great controversy, with debates centered around whether it belongs to the energy band theory of photocatalysis or the screening charge effect of electrochemical catalysis. Due to the formation of different intermediate active-species during two-electron oxygen reduction reaction (ORR) via electro- and photo-catalysis, the key to solving this problem is precisely monitoring the active species involved in ORR during electro-, photo-, and piezo-catalysis under identical condition. Here, a semiconductor material, BiOBr with abundant oxygen vacancies (BOB-OV) was found remarkable catalytic activity in H2O2 production by all three catalytic methods. By employing in-situ electron paramagnetic resonance (EPR) spectroscopy, the H2O2 evolution pathway through piezo-catalysis over BOB-OV was monitored, which showed a similar reaction pathway to that observed in photo-catalytic process. This finding represents solid evidence supporting the notion that piezo-catalytic mechanism of ORR is more inclined towards photo-catalysis rather than electro-catalysis. Significantly, this exploratory conclusion provides insight to deepen our understanding of piezo-catalysis.

The Still Elusive Role of Lightweight Doping in Carbon-Based Electrocatalysts for the Selective Oxygen Reduction Reaction to Hydrogen Peroxide.

The two-electron electrocatalytic oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2) is a valuable alternative to the more conventional and energy-intensive anthraquinone process. From a circularity viewpoint, metal-free catalysts constitute a sustainable alternative for the process. In particular, lightweight hetero-doped C-materials are cost-effective and easily scalable samples that replace - more and more frequently - the use of critical raw elements in the preparation of highly performing (electro)catalysts. Anyhow, their large-scale exploitation in industrial processes still suffers from technical limits of samples upscale and reproducibility other than a still moderate comprehension of their action mechanism in the process. This concept article offers a comprehensive and exhaustive "journey" through the most representative lightweight hetero-doped C-based electrocatalysts and their performance in the 2e- ORR process. It provides an interpretation of phenomena at the triple-phase interface of solid catalyst, liquid electrolyte and gaseous oxygen based on the doping-driven generation of ideal electronic microenvironments at the catalyst surface.

Screening of highly efficient electrocatalysts for hydrogen peroxide synthesis using single transition metal atoms embedded in carbon vacancy fullerene C60

The electrocatalytic two-electron oxygen reduction reaction (2e– ORR) synthesis of hydrogen peroxide (H2O2) is an eco-friendly process. In this study, density functional theory (DFT) and machine learning (ML) were utilized to systematically explore various single metal atoms anchored on mono-vacancy fullerene C60 (TM@C60) for H2O2 production. Among 19 single-atom catalysts (SACs), Sc@C60 and Y@C60 were identified for high the thermodynamic feasibility of H2O2 formation, exhibiting overpotentials of 0.01 V and 0.02 V, respectively. Moreover, competitive four-electron oxygen reduction reaction (4e– ORR) and hydrogen evolution reaction (HER) were significantly suppressed on these two catalysts. The formation of peroxide intermediates on the two screened SACs was identified as key to achieving high activity and selectivity for H2O2 formation. Analysis using ML methods also revealed that the balance between bond strengths and adsorption energies for O2 is crucial in regulating the performance of 2e– ORR. These results offer important insights for electrocatalytic H2O2 production.

Anionic Surfactant-Modulated Electrode-Electrolyte Interface Promotes H2O2 Electrosynthesis.

Conventional strategies for highly selective and active hydrogen peroxide (H2O2) electrosynthesis primarily focus on catalyst design. Electrocatalytic reactions take place at the electrified electrode-electrolyte interface. Well-designed electrolytes, when combined with commercial catalysts, can be directly applied to high-efficiency H2O2 electrosynthesis. However, the role of electrolyte components is equally crucial but is significantly under-researched. In this study, anionic surfactant n-tetradecylphosphonic acid (TDPA) and its analogs are used as electrolyte additives to enhance the selectivity of the two-electron oxygen reduction reaction. Mechanistic studies reveal that TDPA assembled over the electrode-electrolyte interface modulates the electrical double-layer structure, which repels interfacial water and weakens the hydrogen-bond network for proton transfer. Additionally, the hydrophilic phosphonate moiety affects the coordination of water molecules in the solvation shell, thereby directly influencing the proton-coupled kinetics at the interface. The TDPA-containing catalytic system achieves a Faradaic efficiency of H2O2 production close to 100% at a current density of 200mA cm-2 using commercial carbon black catalysts. This research provides a simple strategy to enhance H2O2 electrosynthesis by adjusting the interfacial microenvironment through electrolyte design.

Unravelling the UV/H2O2 process using bioelectrochemically synthesized H2O2 to reuse waste nutrient solution

In this study, waste nutrient solution (WNS) was used as a catholyte in a bioelectrochemical cell to directly produce hydrogen peroxide (H2O2), after which the H2O2- containing WNS was integrated with the downstream UV oxidation process to meet quality standards for reuse. The generated current in the bioelectrochemical cell was successfully utilized at the cathode to produce H2O2 in WNS using a two-electron oxygen reduction reaction with different reaction times. The cathodic reaction time with the highest H2O2 production (504 ± 5.2 mg l−1) was 48 h, followed by that obtained from 24 h (368 ± 4.1 mg l−1), 12 h (158.8 ± 2.4 mg l−1), and 6 h (121.1 ± 4.1 mg l−1) reaction times. During H2O2 generation, calcium, magnesium, and phosphate in the WNS were recovered in the form of precipitates under alkaline conditions. The H2O2-containing WNS was further treated with different UV doses. After UV/H2O2 treatment, excitation-emission matrix and molecular weight distribution analyses demonstrated that aromatic compounds were reduced. Moreover, the gene expressions of sul1 (up to 95.65%), tetG (up to 93.88%), and aadA (up to 95.32%) were clearly downregulated compared with those of a control sample. Finally, a high disinfection efficiency was achieved with higher UV doses, resulting in successful seed germination. Thus, our results indicate that the developed method can be a promising process for reusing WNS in hydroponic systems.

Co-doping regulation on Ni-based electrocatalysts to adjust the selectivity of oxygen reduction reaction for Zn-air batteries and H2O2 production.

Although Ni-based materials are widely used as electrocatalysts, it remains necessary to further explore their selectivity towards the four- or two-electron oxygen reduction reaction (ORR). Herein, it is proposed to synthesize NiO@NCNTs (NCNTs = N-doped carbon nanotubes) using a metal-organic framework (MOF), [Ni(BZIDA)(H2O)]n (NiMOF, BZIDA = benzimidazole-5,6-dicarboxylic acid), as a precursor after calcination with dicyandiamide (DCDA). Regarding NiO@NCNTs, small NiO particles are distributed in NCNTs derived from DCDA homogeneously. NiO@NCNTs act as a typical two-electron electrocatalyst. The H2O2 production rate of NiO@NCNTs reaches 0.5 mol g-1 h-1 at 0.46 V (vs. RHE). After the doping of Co2+ in NiMOF, Co/NiO@NCNTs were synthesized using a similar method, with the four-electron character shown in ORR. A Zn-air battery was assembled by applying Co/NiO@NCNTs as the cathode material. When discharge occurs at 5 and 10 mA cm-2, its specific capacitance reaches 779.3 and 832.2 mA h g-1 with an energy density of 928.6 and 948.5 W h kg-1, respectively. Theoretical calculations suggest a variation in ORR selectivity between NiO@NCNTs and Co/NiO@NCNTs, which results from their different interactions with OOH*. This study demonstrates the effect of the structure on ORR selectivity for Ni-based electrocatalysts.

A Generalized Coordination Engineering Strategy for Single-Atom Catalysts toward Efficient Hydrogen Peroxide Electrosynthesis.

Designing non-noble metal single-atom catalysts (M-SACs) for two-electron oxygen reduction reaction (2e-ORR) is attractive for the hydrogen peroxide (H2O2) electrosynthesis, in which the coordination configuration of the M-SACs essentially affects the reaction activity and product selectivity. Though extensively investigated, a generalized coordination engineering strategy has not yet been proposed, which fundamentally hinders the rational design of M-SACs with optimized catalytic capabilities. Herein, a generalized coordination engineering strategy is proposed for M-SACs toward H2O2 electrosynthesis via introducing heteroatoms (e.g., oxygen or sulfur atoms) with higher or lower electronegativity than nitrogen atoms into the first sphere of metal-N4 system to tailor their electronic structure and adjust the adsorption strength for *OOH intermediates, respectively, thus optimizing their electrocatalytic capability for 2e-ORR. Specifically, the (O, N)-coordinated Co SAC (Co-N3O) and (S, N)-coordinated Ni SAC (Ni-N3S) are precisely synthesized, and both present superior 2e-ORR activity (Eonset: ≈0.80V versus RHE) and selectivity (≈90%) in alkaline conditions compared with conventional Co-N4 and Ni-N4 sites. The high H2O2 yield rates of 14.2 and 17.5moLg-1h-1 and long-term stability over 12h are respectively achieved for Co-N3O and Ni-N3S. Such favorable 2e-ORR pathway of the catalysts is also theoretically confirmed by the kinetics simulations.

Asymmetric structures to switch on the selective oxygen reduction to hydrogen peroxide

Carbon defects coupled with heteroatoms can asymmetrically rearrange the local electronic distribution and coordination environment of active sites, improving the catalytic selectivity and activity of a two-electron oxygen reduction reaction (2eORR). In this study, an asymmetry defective carbon (asy-DC) structure using wolfberry as the carbon source is employed to adjust the charge distribution of active sites with different degrees of asymmetry caused by N→S coordination bonds. The asymmetric region exhibits a considerable positive correlation between the asymmetry degree and adsorption energy for OOH*, presenting a volcano relation between the asymmetry degree and catalytic activity. The optimised asy-DC catalyst exhibits high selectivity and reliable activity after 12 h of stability testing. This study can provide a new reference into the origin of ORR activity and selectivity.

Efficient H2O2 Synthesis through a Two-Electron Oxygen Reduction Reaction by Electrocatalysts.

The two-electron oxygen reduction reaction (2e-ORR) for the sustainable synthesis of hydrogen peroxide (H2O2) has demonstrated considerable potential for local production of this environmentally friendly chemical oxidant on small, medium, and large scales. This method offers a promising alternative to the energy-intensive anthraquinone approach, placing a primary emphasis on the development of efficient electrocatalysts. Improving the efficiency of electrocatalysts and uncovering their catalytic mechanisms are essential steps in achieving high 2e-ORR activity, selectivity, and stability. This comprehensive review summarizes recent advancements in electrocatalysts for in-situ H2O2 production, providing a detailed overview of the field. In particular, the review delves into the design, fabrication, and investigation of catalytic active sites contributing to H2O2 selectivity. Additionally, it highlights a range of electrocatalysts including pure metals and alloys, transition metal compounds, single-atom catalysts, and carbon-based catalysts for the 2e-ORR pathway. Finally, the review addresses significant challenges and opportunities for efficient H2O2 electrosynthesis, as well as potential future research directions.

In situ Production of Hydroxyl Radicals via Three-Electron Oxygen Reduction: Opportunities for Water Treatment.

The electro-Fenton (EF) process is an advanced oxidation technology with significant potential; however, it is limited by two steps: generation and activation of H2O2. In contrast to the production of H2O2 via the electrochemical two-electron oxygen reduction reaction (ORR), the electrochemical three-electron (3e-) ORR can directly activate molecular oxygen to yield the hydroxyl radical (·OH), thus breaking through the conceptual and operational limitations of the traditional EF reaction. Therefore, the 3e- ORR is a vital process for efficiently producing ·OH in situ, thus charting a new path toward the development of green water-treatment technologies. This review summarizes the characteristics and mechanisms of the 3e- ORR, focusing on the basic principles and latest progress in the in situ generation and efficient utilization of ·OH through the modulation of the reaction pathway, shedding light on the rational design of 3e- ORR catalysts, mechanistic exploration, and practical applications for water treatment. Finally, the future developments and challenges of efficient, stable, and large-scale utilization of ·OH are discussed based on achieving optimal 3e- ORR regulation and the potential to combine it with other technologies.

Advanced Nanocarbons Toward two-Electron Oxygen Electrode Reactions for H2O2 Production and Integrated Energy Conversion.

Hydrogen peroxide (H2O2) plays a pivotal role in advancing sustainable technologies due to its eco-friendly oxidizing capability. The electrochemical two-electron (2e-) oxygen reduction reaction and water oxidation reaction present an environmentally green method for H2O2 production. Over the past three years, significant progress is made in the field of carbon-based metal-free electrochemical catalysts (C-MFECs) for low-cost and efficient production of H2O2 (H2O2EP). This article offers a focused and comprehensive review of designing C-MFECs for H2O2EP, exploring the construction of dual-doping configurations, heteroatom-defect coupling sites, and strategic dopant positioning to enhance H2O2EP efficiency; innovative structural tuning that improves interfacial reactant concentration and promote the timely release of H2O2; modulation of electrolyte and electrode interfaces to support the 2e- pathways; and the application of C-MFECs in reactors and integrated energy systems. Finally, the current challenges and future directions in this burgeoning field are discussed.

Visible light-induced photocatalytic degradation of tetrabromobisphenol A on platinized tungsten oxide

In this study, we investigated the degradation of the flame retardant tetrabromobisphenol A (TBBPA) using platinized tungsten oxide (Pt/WO3), synthesized via a simple photodeposition method, under visible light. The results of degradation experiments show a significant enhancement in TBBPA degradation upon surface platinization of WO3, with the degradation rate increasing by 13.4 times compared to bare WO3. The presence of Pt on the WO3 surface stores conduction band electrons, which facilitates the two-electron reduction of oxygen and enhances the production of valence band holes (hVB+) and hydroxyl radicals (●OH). Both hVB+ and ●OH are significantly involved in the degradation of TBBPA in the visible light-irradiated Pt/WO3 system. This was verified through fluorescence spectroscopy employing coumarin as a chemical probe and oxidizing species-quenching experiments. The analysis of degradation products and their toxicity assessment demonstrate that the toxicity of TBBPA-contaminated water is significantly reduced after Pt/WO3 photocatalysis. The degradation rate of TBBPA increased with increasing Pt/WO3 dosage, reached an optimum at a Pt content of 0.5 wt%, but decreased with increasing TBBPA concentration. The decrease in degradation efficiency of Pt/WO3 was minor, both in the presence of various anions and after repeated use. This study proposes that Pt/WO3 is a viable photocatalyst for the degradation of TBBPA in water under visible light.

High efficiency two-step electrocatalysis boosted by dual-motif bridge Co-S-Mo for Heterogeneous Electro-Fenton: Enhanced H2O2 production and fluoxetine elimination synergistic mechanism

Heterogeneous electro-Fenton process is widely recognized as intensive in-situ strategy with environmental compatibility. However, the simultaneous elevation of H2O2 production and exploitation efficiency during electrocatalysis remains a challenge. Here, we propose an innovative dual-site catalyst (MoxCo9−xS8−yNy; MCN) with Mo active motifs for enhanced two-electron oxygen reduction reaction and Co active motifs for enhanced reactive oxygen species generation to promote H2O2 generation and fluoxetine (FLX) degradation. As determined by Mo scaling and application performance, the optimal MCN catalyst provided at least 20 % increase in contaminant elimination within 30 min, reduced the reaction potential by over 1.4 eV, and diminished environmental exposure to toxic products. The optimal MCN@CF cathode efficiently removed approximately 96.7 % FLX and 55.1 % total organic carbon at a pH of 3 within 35 min. The critical dual-motif bridge Co–S–Mo mediated by S could disrupt the synergistic *O and *OOH binding energies, significantly penetrating the catalysis barriers. This work unravels the structural optimization evolution of non-single-site catalysts, providing new solutions to the refractory issues of wastewater decontamination

Tandem Proton Transfer in Carboxylated Supramolecular Polymer for Highly Efficient Overall Photosynthesis of Hydrogen Peroxide.

Proton supply is as critical as O2 activation for artificial photosynthesis of H2O2via two-electron oxygen reduction reaction (2e- ORR). However, proton release via water dissociation is frequently hindered because of the sluggish water oxidation reaction (WOR), extremely limiting the efficiency of photocatalytic H2O2 production. To tackle this challenge, carboxyl-enriched supramolecular polymer (perylene tetracarboxylic acid - PTCA) is elaborately prepared by molecular self-assembly for overall photosynthesis of H2O2. Interestingly, the interconversion between carboxyl as Brønsted acid and its conjugated base realizes rapid proton circulation. Through this efficient tandem proton transfer process, the spatial effect of photocatalytic reduction and oxidation reaction is greatly enhanced with reduced reaction barrier. This significantly facilitates 2e- photocatalytic ORR to synthesize H2O2 and in the meanwhile promotes 4e- photocatalytic WOR to evolve O2. Consequently, the as-developed PTCA exhibits a remarkable H2O2 yield of 185.6 μM h-1 in pure water and air atmosphere under visible light illumination. More impressively, an appreciable H2O2 yield of 78.6 μM h-1 can be well maintained in an anaerobic system owing to in-situ O2 generation by 4e- photocatalytic WOR. Our study presents a novel concept for artificial photosynthesis of H2O2 via constructing efficient proton transfer pathway to enable rapid proton circulation.

Ultra-Rapid Electrocatalytic H2O2 Fabrication over Mono-Species and High-Density Polypyrrolic-N Sites.

Electrocatalytic hydrogen peroxide (H2O2) production via two-electron oxygen reduction reaction (2e--ORR) features energy-saving and eco-friendly characteristics, making it a promising alternative to the anthraquinone oxidation process. However, the common existence of numerous 2e--ORR-inactive sites/species on electrocatalysts tends to catalyze side reactions, especially under low potentials, which compromises energy efficiency and limits H2O2 yield. Addressing this, a high surface density of mono-species pyrrolic nitrogen configurations is formed over a polypyrrole@carbon nanotube composite. Thermodynamic and kinetic calculation and experimental investigation collaboratively confirm that these densely distributed and highly selective active sites effectively promote high-rate 2e--ORR electrocatalysis and inhibit side reactions over a wide potential range. Consequently, an ultra-high and stable H2O2 yield of up to 67.9/51.2molg-1h-1 has been achieved on this material at a current density of 200/120mAcm-1, corresponding Faradaic efficiency of 72.8/91.5%. A maximum H2O2 concentration of 13.47gL-1 can be accumulated at a current density of 80mAcm-1 with satisfactory stability. The strategy of surface active site densification thus provides a promising and universal avenue toward designing highly active and efficient electrocatalysts for 2e--ORR as well as a series of other similar electrochemical processes.

Overcome the trade-off in electro-fenton chemistry for in situ H2O2 generation-activation by tandem CoFe bimetallic single-atom configuration

Dual-atomic catalysts (DACs) demonstrated remarkable potential in addressing key challenges in electro-Fenton (EF) processes. In this study, we synthesized an EF DACs comprising both CoN4 and FeN4 sites, which was achieved a high H2O2 generation rate (1.68 mM−1h−1) and 100 % bisphenol A degradation efficiency via successive two-electron oxygen reduction and one-electron Fenton reactions (2e− ORR + 1e− Fenton). Our findings indicated that the single-atom nitrogen coordination of CoN4 and FeN4 sites plays crucial roles in regulating the adsorption of key intermediates of *OOH and *H2O2. The bimetallic sites independently regulated the binding energies of *OOH on CoN4 (pyrrole-type) for favorable H2O2 generation and its subsequent activation on adjacent FeN4 (pyridine-type). Thus, the dual-site engineering addresses the trade-off of in situ H2O2 generation-activation in EF chemistry, realizing high electron utilization efficiency and fast pollutant degradation toward efficient and sustainable water treatment.

Error Awareness in the Volcano Plots of Oxygen Electroreduction to Hydrogen Peroxide.

Electrocatalysis holds the key to the decentralized production of hydrogen peroxide via the two-electron oxygen reduction reaction (ORR, O2g+2H++e-→H2O2aq). However, cost-effective, active, and selective catalysts are still sought after. While density functional theory (DFT) has already led to the discovery of various enhanced catalysts, it has a severe yet often unnoticed drawback: the ill description of O2 and H2O2. Here, we analyze the impact of the errors in those two species on the most widespread activity plots in the literature, namely free-energy diagrams and Sabatier-type volcano plots. Uncorrected or partially corrected gas-phase energies lead to appreciably different activity plots that may provide inaccurate predictions. Indeed, we show for a variety of electrocatalysts that only when the errors in O2 and H2O2 are corrected can DFT mimic the experiments. In sum, this work provides concrete guidelines to avoid a common pitfall of computational models for electrocatalytic hydrogen peroxide production.

Constructing Photocatalytic Covalent Organic Frameworks with Aliphatic Linkers.

Photocatalytic covalent organic frameworks (COFs) are typically constructed with rigid aromatic linkers for crystallinity and extended π-conjugation. However, the essential hydrophobicity of the aromatic backbone can limit their performances in water-based photocatalytic reactions. Here, we for the first time report the synthesis of hydrophilic COFs with aliphatic linkers [tartaric acid dihydrazide (TAH) and butanedioic acid dihydrazide] that can function as efficient photocatalysts for H2O2 and H2 evolution. In these hydrophilic aliphatic linkers, the specific multiple hydrogen bonding networks not only enhance crystallization but also ensure an ideal compatibility of crystallinity, hydrophilicity, and light harvesting. The resulting aliphatic linker COFs adopt an unusual ABC stacking, giving rise to approximately 0.6 nm nanopores with an improved interaction with water guests. Remarkably, both aliphatic linker-based COFs show strong visible light absorption, along with a narrow optical band gap of ∼1.9 eV. The H2O2 evolution rate for TAH-COF reaches up to 6003 μmol h-1 g-1, in the absence of sacrificial agents, surpassing the performance of all previously reported COF-based photocatalysts. Theoretical calculations reveal that the TAH linker can enhance the indirect two-electron oxygen reduction reaction for H2O2 production by improving the O2 adsorption and stabilizing the *OOH intermediate. This study opens a new avenue for constructing semiconducting COFs using nonaromatic linkers.

Tandem Fields Facilitating Directional Carrier Migration in Van der Waals Heterojunction for Efficient Overall Piezo-synthesis of H2O2.

Piezo-synthesis of H2O2 utilizing sustainable mechanical energy as well as earth-abundant water and oxygen is a green, cost-effective and promising approach. However, achieving simultaneous two-electron water oxidation reaction (2e- WOR) and two-electron oxygen reduction reaction (2e- ORR) faces huge challenges due to insufficient synergistic active sites and slow/messy carrier transfer. Herein, a novel 2D/2D van der Waals (vdW) heterojunction consisting of BiOIO3 and carbon nitride (BIO/CN) was elaborately designed for highly efficient overall H2O2 piezo-synthesis. Theoretical/experimental results revealed that a Z-scheme electron transfer was formed and facilitated by the tandem interfacial electric field and the bulk piezo-polarization field. On this basis, the carriers are efficiently separated while the oxidation/reduction capacity is preserved, thus providing strong driving force for the 2e- WOR and 2e- ORR on BIO and CN, respectively. Furthermore, the kinetic and thermodynamic process of WOR and ORR for H2O2 synthesis improved remarkably. Therefore, BIO/CN exhibits an excellent H2O2 yield of 259.8 μM within 30min in pure water and air atmosphere (without any sacrificial agents and aeration). This study provides a new idea on strategically controlling electron transfer towards high-efficiency H2O2 piezo-synthesis and expands the avenue for developing effective environmental purification materials. This article is protected by copyright. All rights reserved.

Modification of NiSe2 Nanoparticles by ZIF-8-Derived NC for Boosting H2O2 Production from Electrochemical Oxygen Reduction in Acidic Media

The two-electron oxygen reduction reaction (2e− ORR) has emerged as an attractive alternative for H2O2 production. Developing efficient earth-abundant transition metal electrocatalysts and reaction mechanism exploration for H2O2 production are important but remain challenging. Herein, a nitrogen-doped carbon-coated NiSe2 (NiSe2@NC) electrocatalyst was prepared by successive annealing treatment. Benefiting from the synergistic effect between the NiSe2 nanoparticles and NC, the 2e− ORR activity, selectivity, and stability of NiSe2@NC in 0.1 M HClO4 was greatly enhanced, with the yield of H2O2 being 4.4 times that of the bare NiSe2 nanoparticles. The in situ Raman spectra and density functional theory (DFT) calculation revealed that the presence of NC was beneficial for regulating the electronic state of NiSe2 and optimizing the adsorption free energy of *OOH, which could enhance the adsorption of O2, stabilize the O-O bond, and boost the production of H2O2. This work provides an effective strategy to improve the performance of the transition metal chalcogenide for 2e− ORR to H2O2.